This invention relates to an improved catalyst composition useful for the production of vinyl aromatic monomers by the steam dehydrogenation of their alkyl aromatic counterparts. More specifically, the invention relates to new self-regenerative catalysts and a process for using them to obtain more efficient dehydrogenation of alkylbenzenes, alkylbiphenyls, and alkylnaphthalenes.
Self-regenerative catalysts of the prior art have been based primarily on ferric oxide or copper chromite with more or less potassium added, usually in the form of potassium carbonate, to promote the water-gas reaction and thereby to prevent the catalyst from being choked with deposited carbon. Representative prior art showing such catalysts includes U.S. Pat. Nos. 2,870,228; 2,916,531; 3,179,706; and 3,703,593. Some of these patents describe broadly ferric oxide-based catalysts containing as much as 55 percent K.sub.2 CO.sub.3. The well-known Shell 205 dehydrogenation catalyst, for example, contains about 35 percent K.sub.2 CO.sub.3. With gross amounts of potassium promoter such as this, it is possible to use somewhat lower steam to hydrocarbon ratios in the dehydrogenation of alkyl aromatics. With Shell 105 catalyst which contains about 14 percent K.sub.2 CO.sub.3 and using an externally heated reactor, the dehydrogenation of ethylbenzene to styrene can be carried out successfully with a 1:1 steam:hydrocarbon ratio. However, these catalysts based primarily on Fe.sub.2 O.sub.3 have not proven entirely satisfactory for the production of substituted styrenes such as vinyltoluene.
In recent years, improved catalysts of this general type based on a mixture of ferric and zinc oxides have been described, by two of us and another in U.S. Pat. No. 3,205,179 and by one of us an two others in U.S. Pat. No. 3,907,916. In U.S. Pat. No. 3,205,179 and Fe.sub.2 O.sub.3 -ZnO catalyst containing 9 percent K.sub.2 CO.sub.3 is specifically disclosed in connection with the process of catalyst activation by roasting or calcining under particular conditions therein claimed. The patent discloses the use of this catalyst for the steam dehydrogenation of ethylbenzene, with good results obtained at a steam:hydrocarbon feed ratio of 1.9:1. More recent commercial production of styrene with this and other available catalysts typically uses about a 1:1 ratio of steam to hydrocarbon feed. In U.S. Pat. No. 3,907,916 which claims a method for periodically reactivating such catalysts by steaming, Fe.sub.2 O.sub.3 -ZnO catalysts containing up to 22.5 percent K.sub.2 CO.sub.3 are disclosed and one such catalyst is specifically shown to provide good results in the dehydrogenation of ethyltoluene using a steam:hydrocarbon feed ratio of 1.7:1. However, a steam ratio of about 3:1 has been the standard condition in commercial dehydrogenation of ethyltoluene to vinyltoluene.